The present disclosure relates to a fixing device configured to fix a toner image onto a recording medium and to an image forming apparatus including the fixing device.
An electro-photographic image forming apparatus, such as a printer, a copier, a facsimile and a multifunction peripheral, is provided with a fixing device configured to fix a toner image onto a recording medium, such as a sheet of paper. The fixing device fixes the toner image on the sheet by passing the sheet through a fixing nip formed between a fixing member and a pressure member which come in pressure contact with each other.
Such fixing device constructed as described above is provided with a separation member on a downstream side of the fixing nip in a sheet conveying direction in order to prevent the sheet from wrapping around the fixing member after passing through the fixing nip and to steadily separate the sheet from the fixing member. The separation member is often provided so as to keep a predetermined gap from the fixing member in order to avoid the fixing member from being damaged and the separation member from affecting the toner image fixed on the sheet.
Still further, there is a case where the fixing apparatus is configured to adjust a pressure (fixing pressure) between the fixing member and the pressure member at the fixing nip by bringing the pressure member close to or apart from the fixing member in consideration of durability of the fixing member, applicability to various sheets (plain sheet, thick sheet, envelope and the like), resolution of a document or the like. In a case of a thick recording medium such as an envelope for example, the fixing pressure is set to be lower than that of a plain sheet.
When the fixing pressure increases, if a thin cylindrical fixing roller is used as the fixing member, there is a case where the fixing roller is pressed inside and then deflects. Here, in a case when the separation member is positioned such that a gap between the separation member and the fixing roller has a predetermined value in a state in which the fixing pressure is low, i.e., in a state in which the deflection of the fixing roller is small, if the deflection of the fixing roller increases, there arises a problem that the sheet cannot be adequately separated by the separation member because the gap between the fixing roller and the separation member increases. The gap between the fixing roller and the separation member becomes wide at a center portion of the fixing roller in particular. Meanwhile, if the separation member is positioned such that the gap between the separation member and the fixing roller has a predetermined value in a state in which the fixing pressure is high, i.e., in a state in which the deflection of the fixing roller is large, if the deflection of the fixing roller is small, there is a case where the separation member comes into contact with the fixing roller, thus damaging the fixing roller.
Then, there is proposed a fixing device configured to support a sheet-like separation member by supporting members provided at predetermined intervals in a width direction of the sheet. This fixing device is configured to prevent the separation member from interfering with the fixing member by separating the pressure member from the fixing member and also separating the separation member from the fixing member by manipulating a handle at a jammed sheet processing. There is also proposed a fixing device in which a center portion of the separation member is curved into an arc shape protruding toward a sheet conveying direction in consideration that the gap between the fixing member and the separation member varies if the separation member is thermally deformed.
When it is required to lower a conveying speed, for example, when an image of high resolution of 1200 dpi is outputted to a plain sheet, because there a passing period of the sheet through the fixing nip is prolonged and the sheet may receive an excessive amount of heat, there is a demand to feed a sheet through the fixing nip under a low fixing pressure. However, if the separation member is positioned based on the state in which the pressure member is in pressure contact with the fixing member as described above fixing devices, it is unable to accommodate to such requirement and to adjust the fixing pressure suitable for thickness of the sheets.